In a process of manufacturing semiconductor devices, in order to grow a gallium nitride (GaN) based nitride layer, i.e., a GaN-based nitride layer, substrates composed of, usually, sapphire (Al2O3), silicon carbide (SiC), or the like have mainly been used. They are different from the GaN-based nitride layer in terms of their physical properties such as lattice constant and coefficient of thermal expansion. Thus, it is difficult to grow the GaN-based nitride layer of a high quality. Therefore, lots of schemes for growing the GaN-based nitride layer have been presented continually.
The most representative one includes a method of using a buffer layer. In this method, an Al(x)Ga(y)In(z)N (0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1) layer is grown alone or through several combinations at a temperature ranging from 450° C. to 600° C. The growth of the Al(x)Ga(y)In(z)N layer is stopped and the temperature is then increased. Then, Al(x)Ga(y)In(z)N (0≦x≦1,0≦y≦1,0≦z≦1, x+y+z=1) that has been grown at a low temperature becomes nuclei. A GaN-based nitride layer of a high quality is grown using the nuclei as a seed. This buffer layer includes an AlN buffer layer (U.S. Pat. No. 4,855,249), an AlGaN buffer layer (U.S. Pat. No. 5,290,393), an AlInN buffer layer (U.S. Pat. No. 6,508,878) and the like.
However, although the GaN-based nitride layer is grown by this method, there is a problem in that the GaN-based nitride layer has a dislocation density of about 1010 to 1012/cm2.
As an alternative, a buffer layer is not grown on a sapphire substrate at a low temperature as above, but a GaN-based nitride layer semiconductor is grown immediately on a substrate at a high temperature. However, this method has lots of room for improvement.
Meanwhile, researches on a method for growing silicon carbide (SiC) on a sapphire wafer have rarely been made. However, M. C. Luo demonstrated on his report that silicon carbide grown on (0001) sapphire has a 6H structure by means of Raman scattering measurement and X-Ray. Diffraction (XRD) analysis method. However, this experiment is directed to analysis into the structure of a formed silicon carbide layer, but not to a method for fabricating a GaN-based nitride layer having good physical properties on the silicon carbide layer.
Moreover, U.S. Pat. No. 6,242,764 discloses a method in which an AlGaN layer of a high quality is grown on a sapphire or silicon carbide substrate using a single crystalline silicon carbide layer at a high temperature (>1300° C.) as a buffer layer. As in the U.S. Pat. No. 6,242,764, however, if the single crystalline silicon carbide layer is grown on sapphire, mismatch rate becomes great between the two materials. Therefore, this method has disadvantages that a silicon carbide layer having a pretty high thickness (approximately 5 μm) is needed in order to grow the AlGaN layer of a high quality, and the thickly formed silicon carbide layer has a low adhesive strength with sapphire.